Hi,
1. You can do it the joomla way. Duplicate all your products and categories, one per language, as well as the menus of HikaShop and assign the corresponding category to each menu based on the language the menu is assigned to. That might however not be possible to do that if you want to have stock handling not separated per language, and it adds some overhead to the setup of your content in HikaShop.
2. You don't have to buy Falang for the multilingual support. It's only if you want to remove the link/get support. And with some small CSS or PHP modification, you can hide it if you want with the free edition.
3. You can use the normal way for the rest of the website.